import os, time, random, json

# http://ffmpeg.org/ffmpeg-filters.html#xfade
# ffmpeg -i 1.mp4 -i 2.mp4 -f lavfi -i color=Black:640x360 -filter_complex "[0:v][1:v]xfade=transition=circlecrop:duration=1:offset=0.8" -y output.mp4